I was driving my deville on the highway for a while when the "service transmission" alert came on. I hadnt had any issues with hard shifting up until then. When the light came on, the automatic transmission started shifting hard when going into 2nd gear. I ran the codes myself which cleared the code and now it shifts fine. Error code p0734 was the code that came up which indicated slipping in 4th gear (I haven't noticed any issues with fourth gear). Any thoughts?